1. Field of the Invention
The present invention relates to a monolithic ceramic capacitor.
2. Description of the Prior Art
Monolithic ceramic capacitors generally comprise plural dielectric ceramic layers united to one another to form a monolithic body, a plurality of internal electrodes each formed between two adjacent dielectric ceramic layers, and external electrodes formed on opposite sides of said monolithic body and connected to the alternate internal electrodes.
As a dielectric material for monolithic ceramic capacitors, there have widely been used high permittivity dielectric ceramic compositions of a barium titanate system, especially, those comprising a main component of barium titanate and a small amount of a bismuth compound such as bismuth titanate, bismuth stannate, bismuth zirconate, or the like, incorporated therein as a secondary component. Such dielectric ceramic compositions of a barium titanate system are disclosed in various patent specifications. For example, Japanese patent publication No. 55-48644 discloses a dielectric material of a system, BaTiO.sub.3 +Bi.sub.2 O.sub.3.SnO.sub.2 +Nd.sub.2 O.sub.3. Japanese patent laid-open No. 60-31793 discloses a dielectric material of a system, BaTiO.sub.3 +Bi.sub.2 O.sub.3.SnO.sub.2 +CaZrO.sub.3 +MgTiO.sub.3 +CeO.sub.2. Japanese patent publication No. 56-45242 discloses a dielectric material of a system, BaTiO.sub.3 +Bi.sub.2 O.sub.3.ZrO.sub.2 +CeO.sub.2.
The dielectric layers of monolithic ceramic capacitors are generally manufactured by firing the above dielectric material at about 1200.degree. C. Since the internal electrodes are subjected to such a high sintering temperature of the dielectric material, a material for internal electrodes is required to have a high melting point and high resistance to oxidation at high temperatures. To this end, noble metals such as platinum and silver-palladium alloys have been used as a material for internal electrodes.
However, use of such noble metal results in increase of production cost of the monolithic ceramic capacitors. In addition, if any silver-palladium alloy is used as the internal electrode material, it causes migration of silver into the ceramic layers, resulting in lowering of the electrical properties of the capacitors. Further, the internal electrodes of platinum cause increase of equivalent series resistance of capacitors because of low conductivity of platinum.
One idea to solve these problems is to use copper or a copper alloy as a material for internal electrodes as these materials are low in price but high in conductivity. However, it is impossible to use copper or its alloy as an internal electrode material together with the dielectric ceramic compositions of the prior art since copper and alloys thereof have low melting points and are easy to oxidize. If the dielectric ceramic composition is fired in a reducing atmosphere in order to use copper or a copper alloy as a internal electrode material, barium titanate and bismuth oxide in the bismuth compound are reduced during firing, resulting in lowering of the insulating resistance. Thus, it is impossible to use copper or copper alloys as a material for internal electrodes.